This invention relates to a hydraulic brake system with anti-skid control and/or traction slip control for automotive vehicles. Such a brake system has a braking pressure generator with a pressure compensation reservoir and braking pressure control valves by means of which pressure medium can be tapped from the wheel brakes of the controlled vehicle wheels in the braking pressure reduction phase during slip control. It also has an auxiliary pressure supply system connected to the pressure compensation reservoir and switched on during slip control so that during slip control, pressure medium can be supplied into the wheel brakes of the controlled vehicle wheels.
A brake system of this type is described in co-pending U.S. application Ser. No. 884,195 filed July 10, 1986 now U.S. Pat. No. 4,750,788 (corresponding to W. German published patent application No. P 35 27 190). In one embodiment, the braking pressure generator comprises a tandem master cylinder with a vacuum brake booster connected upstream. It also includes an auxiliary pressure supply system which generates a pedal-force-proportional auxiliary pressure during anti-skid control and which generates an uncontrolled auxiliary pressure for traction slip control. Electromagnetic switch-over multi-directional valves connect the wheel brakes to the master cylinder in the rest position and, during control phases to the auxiliary pressure supply system. The motor of the hydraulic pump of the auxiliary pressure supply system is switched on as soon as the system's control electronics recognize, from information gathered by wheel sensors about the rotational behavior of the wheels, that there is a lock-up tendency or excessive traction slip at any one of the wheels. A ball-type seat valve is provided for controlling the auxiliary pressure. This valve is connected to a working chamber of the master cylinder to ensure that--as the braking pressure increases--the valve-induced throttling action in the pressure medium path from the delivery side to the suction side of the hydraulic pump is increased. In the traction slip control phase, a 2/2-way valve, inserted in the pressure medium's circuit, via the control valve, is changed over to lock. Thus, also in case of the master cylinder being unpressurized, the pressure medium path from the pump delivery side to the pump suction side is interrupted, thereby an uncontrolled pressure is built up. There is also provided a switch-over valve which, in case of control, connects the auxiliary pressure supply system instead of the master cylinder to the wheel brakes and also switches over the locking valves which interrupt the pressure medium path to the wheel brakes of the non-driven wheels in this phase.
In the brake system described above and in other known brake systems with anti-skid control and/or traction slip control the braking pressure is controlled in dependence on the rotational behavior of the wheels by means of the electromagnetically operable braking pressure control valves having normally closed outlet valves or outlet paths via which, in the braking pressure reduction phase, a pressure medium path or rather a pressure medium reflux is released from the wheel brakes of the controlled wheels to the braking pressure compensation reservoir of the brake system. This involves the disadvantage that in case of the outlet valves jamming in the opened position or in case of said valves leaking it will be impossible to build up any braking pressure in the appertaining wheel brake. If there is a failure of the auxiliary pressure supply system, e.g., because the pump does not start or does not deliver, it is important to prevent the outlet valves from opening so as to have a sufficient amount of pressure medium remaining in the braking pressure generator in order to provide, at least, an uncontrolled braking action without having recourse to the auxiliary pressure. Thus, a great number of measures are required for supervising the brake system or rather the pressure supply and for rapid recognition of any malfunction as well as for switching off control, completely or in part.